High resolution optical spectrometers typically require highly dispersive optical elements. One example of such optical element is a virtually imaged phase array (VIPA), which is an optical component that has a very high angular dispersion D=dθ/dλ, where θ is a dispersion angle at which light of wavelength λ is dispersed by the VIPA. A conventional VIPA includes two parallel surfaces, one of which typically has a highly-reflective coating, and the other is partially-reflective. The highly-reflective surface has an input zone 6 that may have an anti-reflection coating through which input light may enter the VIPA at an angle, to be reflected back and forth across a gap between the highly and partially reflective surfaces, gradually leaking out through the partially reflective surface. Because the two reflective surfaces are highly parallel, the output beams have a well-defined phase relationship, which interference in a focal plane of a lens results in a strong angular dispersion that enables the use of the VIPA in a spectrometer.
One limitation of a VIPA is a relatively small free spectral range (FSR), which is typically only about 50-80 times greater than the spectral resolution of the VIPA. If two wavelengths incident on the VIPA are separated by one FSR, their angular locations at the detector will be identical. In order to broaden the overall operating range of the spectrometer beyond one VIPA FSR a second dispersive element in a cross-orientation with the VIPA may be added after the VIPA to allow separation of these otherwise overlapping wavelengths. This second dispersive element could be a dispersing prism or a diffraction grating.
The overall spectral resolution of a VIPA-based spectrometer is defined not only by the spectral resolution of the VIPA, but also by the pixel size of a camera used to detect the dispersed light, the size of the pixel array of the camera, and the focal length of a lens that focuses the dispersed light upon the camera. Typically there is a trade-off between the spectral resolution and the measurement spectral window of the spectrometer, so that a spectrometer with a greater spectral resolution will likely have a narrower measurement spectral window.
Thus there is a need for an optical spectrometer that combines a high spectral resolution with a broader operating wavelength range.